1. Field of the Invention
The present invention relates to motor-driven tools, in particular hand-held motor-driven tools such as electric drills and electric drivers.
2. Description of Background Information
Hand-held motor-driven tools are in general constructed as shown in FIGS. 15(a) and 15(b) such that a motor 2 is housed in the rear of the cylindrical housing 1 connected at its middle to a grip 4. A switch trigger 50 and a switch that is opened and closed by manipulating the switch trigger 50 are located on grip 4. The motor 2, as shown in FIG. 15(a), has a centrifugal exhaust fan 8 mounted on the rear end of rotor 28 from which an output shaft 21 protrudes forwardly. An air intake opening 16 is formed in the front outer periphery of the housing 1 and an exhaust opening 18 is formed in the rear peripheral surface. An intake air opening 23a is provided in the rear end surface of the casing 25 of motor 2, and an exhaust air opening 23b is provided in the rear peripheral surface of the casing 25. Air sucked into housing 1 through air intake opening 16 in the front peripheral surface of housing 1 passes through the intake air opening 23a at the rear end of motor 2 and is sent into motor 2, then passes through exhaust opening 18 of housing 1 from exhaust air opening 23b of motor 2 and is exhausted as cooling air. W1 in FIG. 15(a) indicates intake air and W2 indicates exhaust air. Grip 4 is connected midway along the cylindrical housing in consideration of weight balance, and a connecting terminal connected to the power source through a switch on the motor 2 is mounted on the rear side, rather than on the front where output shaft 21 protrudes.
In this case, the air directed toward the intake air openings 23a on the rear of motor 2 from the intake openings 16 on the housing 1 passes through a long, narrow route between the inner peripheral surface of the housing 1 and the outer surface of the motor 2, and further must cross paths with the exhausted air exiting from exhaust air openings 23b on the motor 2. As a result, not only does cooling air sucked in through intake openings 16 of housing 1 flow intake air through openings 23a at the rear of motor 2, but heated air between the motor 2 and the housing 1 also flows through intake air openings 23a. Because there is no cool air flowing along the axis inside the motor, it is difficult to cool the inside of the motor 2, and hot air easily accumulates between the motor 2 and the housing 1. Elements through which current flows, such as the coil, commutator, and brush inside motor 2 which are the largest generators of heat, are not cooled sufficiently. This causes a reduction in the output capacity of motor 2 by raising the electrical resistance. Further, when using these motor-driven tools, the temperature of the outer shell of the housing 1 rises within a short time period, and the portion where the hand grips the tool below the motor 2 and the connection between the cylindrical housing and the grip 4 as well as the rear end of housing 1 where the hand is often in contact when the tool is in use become hot, and the air exhausted from exhaust openings 18 strikes the hand, causing discomfort to the user.
If a motor 2 in which an intake opening is provided in the front end also (not shown) is used, cooling air can pass through the inside of motor 2, but because a decelerator 3 for decelerating the output of motor 2 is connected to the front end of motor 2 in motor-driven tools, such intake opening would either be closed off by the decelerator 3, or the opening would be extremely small due to the need to make the structure compact, and it would be insufficient as a route for cooling air.